Calibration standards for profilometers and methods of producing them

Abstract: To allow faithfully the quantitative interpretation of the measuring results obtained by scanning force microscopes (STM) or atomic force microscopes (AFM) the probe tips used have to be exactly characterized before and after measuring since their size and shape may change during the measuring procedure. If the tips are cone-shaped, their diameter and their cone angle have to be known accurately. Described are calibration standards for profilometers, especially for STMs and AFMs, which are of high accuracy and which allow calibration measurements without frequently removing the probe tips. Methods of producing these calibration standards are shown and examples are given for the use of the calibration standards for measuring features in the sub-nanometer range or for calibrating profilometers.

Description: The present invention relates to calibration standards for profilometers, especially for scanning force microscopes (STM) and for atomic force microscopes (AFM), to methods of producing said calibration standards and to methods for measuring features in the sub-nanometer range or calibrating profilometers with said calibration standards. Probe microscopy like scanning force microscopy and atomic force microscopy at the moment is changing from a pure qualitative microscopy to a more and more also quantitative method. How faithfully a probe microscope shows the surface topography depends strongly on the size and shape of the ultrafine probe tip. To allow the quantitative interpretation of the measuring results the probe tips used for measuring have to be exactly characterized before and after measuring, since the shape of the probe tips may change during the measuring procedure. If cone-shaped tips are used the diameter and the cone angle have to be accurately known. Common solutions also focus on the single crystal silicon technology. In SPIE, Vol. 661 (1986), pp. 402 - 407, "Film thickness and refractive index Standard Reference Material calibrated by ellipsometry and profilometry" G. A. Candela et al. describe a standard reference material (SRM) consisting of a silicon wafer as base plate with a silicon dioxide film of uniform thickness. The patterns of windows in the oxide under a chromium overlayer can be used for interferometric measurements and the uncovered portion in the oxide is preferred for stylus profilometry measurements with a mechanical depth very nearly the same as the oxide thickness. The depth, however, is not exactly the same as the oxide thickness due to native oxide forming in the windows which affects the accuracy of the measurement. In J. Appl. Phys. 74 (9), 1 November 1993, pp. R83 - R109, "Dimensional metrology with scanning probe microscopes" J. E. Griffith and D. A. Grigg proposed an array of pillars with undercut sidewalls for probe characterization (Schematic diagram of a method for characterizing scanning probe tips with the probe microscope, Fig. 32, p. R101). They found out that unfortunately the shape of the probe may change during a scan and that it is important to check the probe often. Frequently removing the probe for inspection, e.g. in an SEM, however, is impractical, especially if the scanning is performed in a controlled ambient or under vacuum. In most SEM inspection cycles the probe will be additionally contaminated and therefore its dimensions will be changed inherently. The undercut of their probe characterizer (Fig. 32) serves two purposes: first it produces a cusp at the upper edge where the proximal point will hang as the probe scans over it and in addition the wall of the pillar is kept away from the probe, so that attractive forces between the probe and sidewall are kept to a minimum. To measure the total width of the probe, the width of the pillar or hole scanned must be determined by other means. As can be seen in Fig. 35, p. R103, the STM-image of the polysilicon pillars used for probe characterization shows no overhang, and the roughness of the tops of the polysilicon pillars is a significant source of error in the probe shape measurement. As a result of their characterization efforts they state that calibration of a probe for use in steep topography requires a standard reference with steep topography, and that in fabricating reference standards for optical linewidth measurements the uncertainty is determined by the roughness of the line edges (p. R107, second paragraph). They also state that their inability to fabricate structures with the desired precision is a serious problem. To satisfy the gauge maker's rule which holds that measurements must be made to within 10 % of the smallest dimension encountered, they resume that calibration artifacts will have to be controlled at the atomic level. To carry that calibration over to the test structure it may be necessary to control the probe at the atomic level too. In their conclusion they point out that the biggest challenges in probe metrology are in the probe-sample interaction and in calibration, since in their opinion fabricating probes and reference materials for calibration poses similar difficulties (p. R107, right col.). In "TopoMetrix, TI Standardizing AFM for Semiconductor Metrology", 38/Semiconductor International, October 1993, it is to be read that currently there are no industry-accepted standards for the calibrationof AFM in applications such as Z-height measurement. According to this short article this limits the use of AFM for in-process metrology in lithography applications. It is therefore an object of the present invention to provide calibration standards of high accuracy which overcome the prior art deficiencies and which allow calibration measurements without frequently removing the probe tips. This object is achieved with the calibration standard according to claim 1. The invention also comprises a method of producing such calibration standard according to claim 4, a method of calibrating the Z-linearity of profilometers according to claim 9 and the use of this standard for measuring features in the sub-nanometer range, according to claim 8. Ways of carrying out the invention are described in detail below with reference to drawings showing only specific embodiments in which: Figs. 1a - 1c show a calibration standard according to this invention and the process steps to produce this standard; Fig. 2 shows another embodiment of the present invention; Fig. 3 explains the depth and width of the grooves in Figs. 1 and 2; Fig. 4 shows a tip scanning over a calibration sample. Fig. 5 gives the line scan results obtained when using a calibration standard according to Fig. 1c. The process sequence for manufacturing the calibration standard of Fig. lc starts in Fig. la where a supporting structure 1 of single crystal material, preferably a wafer of silicon, has been provided. Most preferably, this silicon wafer is a wafer with (100)-orientation. A masking layer 4, typically comprising SiO 2 or Si3N4, is deposited on the surface of the supporting structure 1, followed by the deposition of a photoresist layer (not shown). The photoresist layer and the masking layer 4 are structured using known lithography steps in a way that the remaining portion of the masking layer 4 forms stripes on the silicon wafer, said stripes being oriented in the (110)-direction. With the structured masking layer 4 as mask grooves 2 are etched into the silicon wafer by anisotropically etching it. The etched grooves 2 are V-shaped all showing a constant sidewall angle of 54.7° and a high sidewall quality. In Fig. 1b the sidewalls of the grooves 2 are shrunk by applying suitable isotropic etching or a thermal oxidation followed by a highly selective removal of the oxide which also may be done by etching. The thermal oxidation of the silicon material of the supporting structure 1 consumes a special amount of silicon and thus the (111)-oriented sidewalls of neighboured grooves start approaching each other. Shrinking of the sidewalls continues until the facing sidewalls 3a, 3b of the grooves 2 form a sharp blade 3. With this process sequence the sharpness of the blades 3 may easily be controlled, since slight changes in the endpoint of the shrinking process do not affect the quality of the blades, they only are not of the same height. Then the mask layer 4 is removed, in the case that the oxidation/etching alternative has been carried out together with the oxide, and Fig. 1c) shows the calibration standard with extremely sharp blades 3 of about 10 nm in lateral diameter. Depending on the quality of the single crystal material used for the supporting structure the V-grooves are extremely parallel, at least for several tens of micrometer. This process sequence allows the reproducible production of the blades having a diameter of a few nanometers. The blades should have a length of several tens of micrometer to allow several calibration measurements at different locations. With these multiple measurements random defaults caused as e.g. by particles may be eliminated. The groove structure thus is also suitable for the calibration of the x,y scan orthogonality and for the scan linearity. Fig. 5 gives the line scan results obtained for'a 2D tip when using a calibration standard according to Fig.1 c). The lateral diameter of the blade 3 may be defined with a SEM or TEM with an absolute accuracy of at least +/- 5%, which causes an inaccuracy of about +/- 0.5 nm when the blade itself has a lateral dimension of about 10 nm. This example shows how important it is to provide ultra sharp blades, since calibration structures with about 1 micro-meter dimensions could not be measured with the same accuracy, the example given offers only +/- 50 nm accuracy. After profiling the sharp blade 3 with a tip 5 with e.g. an atomic force microscope or a tunneling force microscope, the known lateral dimension of the blade has to be subtracted from the measured value to obtain the diameter of the tip 5. These blades may be used for defining cone shaped tips as well as tips with flared apex used in the 2D atomic force microscopy. (See Fig. 2c and corresponding description in European Patent 0 413 040). Another preferred embodiment is shown in Fig. 2. In the supporting structure 1 there is an array of V-shaped grooves 2a, 2b, 2c, 2d of different sizes. The manufacturing process is similar to that described in Figs. 1a - 1c. The width W of a groove is defined lithographically with W ranging from about 10 nm to a few hundred micrometers. The maximum width is only limited by the thickness of the supporting structure 1, preferably a silicon wafer. The relation between width W and depth D of a groove is explained in Fig.3. By anisotropically etching a silicon wafer with an (100)-orientation (111)-grooves with respect to the (100)-plane and with a constant sidewall angle of 54.7° are provided. The depth D to width W ratio is defined by: D = 0.706 W Scanning this calibration standard with an ultrafine profilometer tip 5 as shown in Fig. 4 measures the different Wi and Di values of the i grooves 2a, 2b, 2c, 2d with each Di = 0.706 Wi. Without the need of changing the calibration standard these measurements allow to define the Z-linearity of a profilometer with the highest accuracy in a very simple manner. The calibration standard as described in Fig. 2 may also be used for calibrating the z amplitude. This is only possible with the lateral X amplitude being already defined by another method and with an ultrafine tip 5 showing a smaller cone angle and radius of'curvature than the calibration standard itself. There already exist tips with these properties. The manufacturing steps described in connection with the calibration standards of Figs. 1 and 2 are all known microfabrication processes which allow a reproducible and cost effective production of the calibration standards.

Claims: Calibration standard used with a scanning probe microscope for calibrating probe tips, especially for STMs and AFMs, or for measuring features in the subnanometer range comprising a supporting structure (1) of single crystal material which comprises silicon having a (100)-orientation with at least two grooves- (2, 2a, 2b, 2c, 2d), and wherein said at least two grooves (2, 2a, 2b, 2c, 2d) are V-shaped grooves located in close relationship to each other in a way that their facing sidewalls (3a, 3b) having a (111)-orientation form a sharp blade (3) of about 10 nm in lateral diameter. Calibration standard of claim 1, wherein said V-shaped grooves (2, 2a, 2b, 2c, 2d) all have the same width to depth ratio. Calibration standard of claim 1, wherein said V-shaped grooves (2, 2a, 2b, 2c, 2d) comprise an array of V-shaped grooves of different sizes. Method of producing a calibration standard according to any of the claims 1 to 3 comprising the steps: providing a supporting structure (1) of single crystal material which comprises silicon having a (100)-orientation depositing a masking layer (4) on said supporting structure (1) depositing a photoresist layer on said masking layer (4) structuring said photoresist layer (not shown) and said masking layer (4) and anisotropically etching grooves (2) into said supporting structure (1) with said masking layer (4) serving as etch mask. Method of claim 4 further comprising the steps of shrinking the sidewalls of said grooves (2) until their facing sidewalls (3a, 3b) form a sharp blade (3) and removing the structured masking layer (4). Method of claim 5 wherein said shrinking step comprises etching or an oxidizing step followed by an etching step. Method of any of the claims 4 to 6 wherein said masking layer (4) comprises SiO 2 . Method for measuring features in the sub-nanometer range, especially the-diameter of ultrafine tips, using a calibration standard according to claims 1 and 2 comprising providing an ultrafine silicon tip (5) profiling said sharp blade (3) with said ultrafine silicon tip (5) and calculating the diameter of said ultrafine silicon tip (5) by deconvoluting the known blade (3) diameter from the value obtained as a result of said profiling step. Method for calibrating the Z-linearity of profilometers using a calibration standard according to claim 3 comprising providing an ultrafine silicon tip (5) profiling said array of V-shaped grooves (2, 2a, 2b, 2c, 2d) of different sizes with said ultrafine silicon tip (5) and thus measuring the widths W of said V-shaped grooves (2, 2a, 2b, 2c, 2d) and calculating the depths D of said V-shaped grooves according to the formula D = 0.706 x W.

Inventors: BAYER THOMAS, GRESCHNER JOHANN DR, MARTIN YVES, MEISSNER KLAUS, WEISS HELGA, BAYER, THOMAS, GRESCHNER, JOHANN, DR., MARTIN, YVES, MEISSNER, KLAUS, WEISS, HELGA, MARTIN, YVES, DEPT. 433D, IBM CORP.
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES
Application year: 1994
Main classification: G01B1
IPC codes: G01B7, G01N27, G01N37, H01L21, G01N1, G01N13, G01B1